The present invention relates to a method for the preparation of carbon fibers. More particularly, the invention relates to an efficient and improved method for the preparation of pitch-based carbon fibers having an extremely high knot strength and outstandingly high tensile strength.
Carbon fibers are highlighted in recent years as a class of important fibrous materials having high tensile strength and elastic modulus despite their lightness so that they are widely used in a rapidly growing quantity as a base material or a resin-reinforcing material in a variety of application fields including parts of aircrafts and automobiles, sporting goods and the like.
Carbon fibers are classified into two classes of so-called PAN-based ones and pitch-based ones dependinq on the starting material for the preparation thereof. The PAN-based carbon fibers are prepared from polyacrylonitrile fibers as the starting material and characterized by their high tensile strength and intermediate elastic modulus. For example, PAN-based carbon fibers may have an elastic modulus of about 400 GPa at the highest after a heat treatment at 2000.degree. C. or above. PAN-based carbon fibers, however, have disadvantages that it is an inherently difficult matter that they are imparted with an extremely high elastic modulus because PAN-based carbon fibers are poorly graphitizable so that the degree of graphitization cannot be high enough in addition to the relatively high costs as compared with pitch-based carbon fibers.
On the other hand, pitch-based carbon fibers are economically advantageous in respect of the low costs because the starting material thereof is an inexpensive carbonaceous pitch. In particular, an extremely high elastic modulus of around 800 Gpa can be obtained in graphitized pitch-based carbon fibers prepared from a liquid-crystalline mesophase pitch and heat-treated at about 3000.degree. C. Though advantageous in respect of the extremely high elastic modulus, pitch-based carbon fibers are not quite satisfactory when high-strength fibers or high-elongation fibers are desired.
It is noted that carbon fibers are required to be fully pliable when carbon fibers are used as a base material of various kinds of composite materials or woven or knit fabrics are prepared therefrom. Accordingly, it is industrially highly desirable that pitch-based carbon fibers, having economical advantages, are imparted with improved tensile strength and knot strength as a measure of the pliability. Thus, it is eagerly desired to develop a method for the preparation of pitch-based carbon fibers having greatly improved tensile strength and knot strength.
The manufacturing process of pitch-based carbon fibers usually includes the steps of melt-spinning of a carbonaceous pitch into pitch fibers, infusibilization of the pitch fibers and carbonization of the infusibilized pitch fibers. Various attempts and proposals have been hitherto made for improvement of each of these steps. As to the infusibilization treatment of pitch fibers, for example, (1) Japanese Patent Publication No. 48-42696 and Japanese Patent Kokai No. 55-90621, No. 58-53085 and No. 60-259629 teach a method in which pitch fibers are heated in an atmosphere of air containing nitrogen dioxide NO.sub.2, (2) Japanese Patent Kokai No. 63-120112 teaches a method in which carbon fibers of high elastic modulus can be prepared at a lower temperature than in the prior art methods by first selectively infusibilizing the surface layer alone of the pitch fibers and enhancing the crystallinity in the core portion of the fibers, (3) Japanese Patent Kokai No. 63-145419 teaches a method according to which carbon fibers of high strength can be prepared by the infusibilization treatment for a relatively long time at a low temperature of 200.degree. C., (4) Japanese Patent Kokai No. 63-264917 teaches a method for the infusibilization of pitch fibers in which the length of time taken for the treatment can be shortened when the treatment is conducted at a temperature not exceeding 350.degree. C. in an atmosphere of an oxygen-enriched gas containing at least 30% by volume of oxygen, and so on.
These prior art proposals relative to the infusibilization treatment of pitch fibers, however, are not always quite satisfactory from the standpoint of achieving the above mentioned object of the invention. For example, the method (1) is ineffective as a method for the preparation of high-performance carbon fibers since the object of the improvement is directed to the production efficiency. Each of the methods (2) to (4) is indeed effective in obtaining carbon fibers of high strength or high elastic modulus but almost no improvement can be expected thereby in respect of the pliability of the fibers.
As to the step of spinning of a molten carbonaceous pitch material, it is known that a pitch fiber may have a specific internal structure by controlling the spinning conditions. The structure of the carbon fibers disclosed in Japanese Patent Kokai No. 60-238520 is radial in the surface layer portion and onion-like in the core portion. No substantial improvements, however, can be obtained in these carbon fibers having a modified structure in respect of the pliability of the fibers.